New Sequence/Thread checking API.

base/threading/thread_checker.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_THREADING_THREAD_CHECKER_H_
 #define BASE_THREADING_THREAD_CHECKER_H_
 
 #include "base/logging.h"
 #include "base/threading/thread_checker_impl.h"
 
+// ThreadChecker is a helper class used to help verify that some methods of a
+// class are called from the same thread (for thread-affinity).
+//
+// Use the macros below instead of the ThreadChecker directly so that the unused
+// member doesn't result in an extra byte (four when padded) per instance in
+// production.
+//
+// Usage of this class should be *rare* as most classes require thread-safety
+// but not thread-affinity. Prefer base::SequenceChecker to verify thread-safe
+// access.
+//
+// Thread-affinity checks should only be required in classes that use thread-
+// local-storage or a third-party API that does.
+//
+// Prefer to encode the minimum requirements of each class instead of the
+// environment it happens to run in today. e.g. if a class requires thread-
+// safety but not thread-affinity, use a SequenceChecker even if it happens to
+// run on a SingleThreadTaskRunner today. That makes it easier to understand
+// what would need to change to turn that SingleThreadTaskRunner into a
+// SequencedTaskRunner for ease of scheduling as well as minimizes side-effects
+// if that change is made.
+//
+// Usage:
+//   class MyClass {
+//    public:
+//     MyClass() {
+//       // It's sometimes useful to detach on construction for objects that are
+//       // constructed in one place and forever after used from another
+//       // thread.
+//       DETACH_FROM_THREAD(my_thread_checker_);
+//     }
+//
+//     ~MyClass() {
+//       // ThreadChecker doesn't automatically check it's destroyed on origin
+//       // thread for the same reason it's sometimes detached in the
+//       // constructor. It's okay to destroy off thread if the owner otherwise
+//       // knows usage on the associated thread is done. If you're not
+//       // detaching in the constructor, you probably want to explicitly check
+//       // in the destructor.
+//       DCHECK_CALLED_ON_VALID_THREAD(my_thread_checker_);
+//     }
+//
+//     void MyMethod() {
+//       DCHECK_CALLED_ON_VALID_THREAD(my_thread_checker_);
+//       ... (do stuff) ...
+//     }
+//
+//    private:
+//     THREAD_CHECKER(my_thread_checker_);
+//   }
+
+#if DCHECK_IS_ON()
+#define THREAD_CHECKER(name) base::ThreadChecker name
+#define DCHECK_CALLED_ON_VALID_THREAD(name) DCHECK((name).CalledOnValidThread())
+#define DETACH_FROM_THREAD(name) (name).DetachFromThread()
+#else  // DCHECK_IS_ON()
+#define THREAD_CHECKER(name)
+#define DCHECK_CALLED_ON_VALID_THREAD(name)
+#define DETACH_FROM_THREAD(name)
+#endif  // DCHECK_IS_ON()
+
 namespace base {
 
 // Do nothing implementation, for use in release mode.
 //
-// Note: You should almost always use the ThreadChecker class to get the
-// right version for your build configuration.
+// Note: You should almost always use the ThreadChecker class (through the above
+// macros) to get the right version for your build configuration.
 class ThreadCheckerDoNothing {
  public:
   bool CalledOnValidThread() const WARN_UNUSED_RESULT {
     return true;
   }
 
   void DetachFromThread() {}
 };
 
-// ThreadChecker is a helper class used to help verify that some methods of a
-// class are called from the same thread. It provides identical functionality to
-// base::NonThreadSafe, but it is meant to be held as a member variable, rather
-// than inherited from base::NonThreadSafe.
-//
-// While inheriting from base::NonThreadSafe may give a clear indication about
-// the thread-safety of a class, it may also lead to violations of the style
-// guide with regard to multiple inheritance. The choice between having a
-// ThreadChecker member and inheriting from base::NonThreadSafe should be based
-// on whether:
-//  - Derived classes need to know the thread they belong to, as opposed to
-//    having that functionality fully encapsulated in the base class.
-//  - Derived classes should be able to reassign the base class to another
-//    thread, via DetachFromThread.
-//
-// If neither of these are true, then having a ThreadChecker member and calling
-// CalledOnValidThread is the preferable solution.
-//
-// Example:
-// class MyClass {
-//  public:
-//   void Foo() {
-//     DCHECK(thread_checker_.CalledOnValidThread());
-//     ... (do stuff) ...
-//   }
-//
-//  private:
-//   ThreadChecker thread_checker_;
-// }
-//
-// Note that, when enabled, CalledOnValidThread() returns false when called from
-// tasks posted to SingleThreadTaskRunners bound to different sequences, even if
-// the tasks happen to run on the same thread (e.g. two independent TaskRunners
-// with ExecutionMode::SINGLE_THREADED on the TaskScheduler that happen to share
-// a thread).
-//
-// In Release mode, CalledOnValidThread will always return true.
+// Note that ThreadCheckerImpl::CalledOnValidThread() returns false when called
+// from tasks posted to SingleThreadTaskRunners bound to different sequences,
+// even if the tasks happen to run on the same thread (e.g. two independent
+// SingleThreadTaskRunners on the TaskScheduler that happen to share a thread).
 #if DCHECK_IS_ON()
 class ThreadChecker : public ThreadCheckerImpl {
 };
 #else
 class ThreadChecker : public ThreadCheckerDoNothing {
 };
 #endif  // DCHECK_IS_ON()
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_CHECKER_H_